Dear all,

I' m trying to make a model in VUMAT for shape memory alloy based on Lagoudas 's thermomechanical constitutive model. It seems that It's necessary an iteration to evaluate the thermoelastic prediction.

My questions are : is it possible to use an iterative loop in the VUMAT or it's better to avoid it??Could someone suggest me a different way so that I can avoid the iterative loop?

Many thanks,

Cece

Concrete is currently the most important material in the building industry. However, it is very weak in tension,  compared to its strength in compression. To overcome this problem, prestressed concrete is usually used.  Prestressed concrete is plain concrete with reinforcement of steel, polymers or, in this case, shape memory alloys. The prestressing is usually introduced by applying tension to the reinforcement in the concrete members. Consequently, initial compressive stresses are transmitted to the concrete matrix; the application of permanent  compressive stress increases the apparent tensile strength of the concrete, since upon tensile loading, the compressive stresses must first be nullified.

With the increasing use of shape memory alloys in recent years, it is important to investigate the effect of cracks. Theoretically, the stress field near the crack tip is unbounded. Hence, a stress-induced transformation occurs, and the martensite phase is expected to appear in the neighborhood of the crack tip, from the very first loading step. In that case, the crack tip region is not governed by the far field stress, but rather by the crack tip stress field. This behavior implies transformation toughening or softening.

Stent and Nitinol have revolutionized the medicine.  In past decades, guidewires, stents, filters and many minimumly invasive devices and implants are made of Nitinol and they proved to be very successful.

However, the fatigue behavior of Nitinol has not been well understood.  As a consequences, many stent fractures have been observed in-vivo.  Below is a list of misconcepts that may contribute to the widely observed in-vivo fractures on Nitinol stents:

Smart materials have received much attention in recent years, especially due to their various applications in smart structures, medical devices, actuators, space and aeronautics. Among these
materials, shape memory alloys exhibit extremely large, inelastic, recoverable strains (of the order of 10%), resulting from transformation between austenitic and martensitic phases. This
transformation may be induced by a change, either in the applied stress, the temperature, or both.

Xi Wang, Yves Bellouard, Joost J. Vlassak

Published in Acta Materialia 53 (2005) p4955-4961.

Abstract — We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. This technique has the advantage that shape memory properties can be spatially distributed as required by the application. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation.

This micrograph indicates the nulceation and growth mechanism in the crystallization of amorphous near-equiatomic NiTi films. The crystal nucleates homogenously inside the bulk of the film, and quickly consume most of the film thickness, and then grows laterally in a two-dimensional growth mode. Heterogeneous nucleation at an interface was not observed due to the composition shift at those locations caused by interfacial reaction.

Source: Xi Wang, Joost.J. Vlassak, Crystallization Kinetics of Amorphous NiTi Shape Memory Alloy Thin Films, Scripta Materialia, 54, 925-930 (2005). (see attachment for preprint of the paper)